Such a steering wheel unit is known for example from the earlier German Patent Application 100 20 085.0 of Apr. 22, 2000, and has a connection hub which may be non-rotatably connected with a steering shaft of a steering device. The steering wheel unit has, in addition, a steering wheel rim which is non-rotatably connected with the connection hub by means of spokes. Furthermore, a central piece is provided, which usually has an impact body and also an airbag unit. The central piece is mounted by means of a spacer element to the connection hub so as to be rotatable relative to the steering shaft and the steering wheel rim. This spacer element has a first end which is non-rotatably connected with the central piece, and has a second end which is rotatably mounted to the connection hub. Furthermore, the spacer element is constructed as a crash element which fails after a predetermined failure value of a force driving the central piece towards the connection hub. The generic steering wheel unit has, in addition, a coupling device which arrests the central piece with respect to a rotational movement of the steering wheel rim. In the known steering wheel unit, this coupling device is formed by a magnetic force mechanism with magnet arrangements which are integrated into the structure of the steering wheel unit.
From DE 43 28 562 C1 there is known a further steering wheel arrangement, which has a central piece which is rotatably mounted relative to the steering shaft and the steering wheel rim, and is arrested by means of a coupling device with respect to a rotational movement of the steering wheel rim. In this steering wheel unit, the coupling device is formed by a toothed-wheel gearing with bevel gears, a carrier plate, on which the bevel gears are mounted, being driven by rotational adjustments of the steering wheel rim.
In the case of a crash, it may happen that the driver of the vehicle strikes onto this steering wheel unit, generally firstly hitting the steering wheel rim projecting furthest into the interior of the vehicle. Usually, this steering wheel rim is constructed as a crash element by means of a corresponding shaping of its spokes, so that the spokes yield under an absorption of impact energy, with the steering wheel rim moving towards the connection hub. Such forward movement of the steering wheel rim leads to the driver of the vehicle hitting the central piece in a chronologically staggered manner during the impact. As the spacer element, by means of which the central piece is supported on the connection hub, also forms a crash element, this support also fails in the case of sufficiently great forces.
This type of construction has the result that with the impact of the driver of the vehicle onto the steering wheel unit, firstly a first force peak is formed, when the forces transferred from the vehicle driver to the steering wheel rim build up until the failure of the spokes. Furthermore, with a chronological delay, a second force peak is formed, when the forces transferred from the vehicle driver to the central part increase until the failure of the spacer element. As each force peak increases the effective deceleration forces on the vehicle driver, it is desired to reduce or eliminate the second force peak. A reduced or absent second force peak can reduce the risk of injury on impact of the vehicle driver onto the steering wheel unit, whereby the human protection effect of the steering wheel unit, designed as a safety component, is improved.
The present invention deals with the problem of further developing a steering wheel unit of the type initially mentioned, with regard to its human protection effect.